1. Field of the Invention
This invention relates to the use of optically transparent beads in photothermographic and thermographic elements having emulsion coatings of uniform optical density which are easily transported in an imaging apparatus.
2. Background of the Invention
The increasing availability and use of semiconductor light sources, such as laser diodes which emit in the visible and particularly in the red and infrared region of the electromagnetic spectrum, have led to the need for photothermographic and thermographic elements that have the ability to be efficiently exposed by laser imagesetters, light emitting diodes, or laser imagers and which have the ability to form sharp images of high resolution and sharpness. In addition, semiconductor light sources have allowed the design of compact automated equipment which increases the productivity of the imaging process, especially in medical diagnostic and graphic arts applications. The use of heat-developable elements eliminates the use of wet processing chemicals which provides a simpler, environmentally friendly system.
Silver halide-containing, photothermographic imaging materials (i.e., heat-developable photographic elements) processed with heat, and without liquid development, have been known in the art for many years. These materials are also known as "dry silver" compositions or emulsions and generally comprise a support having coated thereon: (1) a photosensitive material that generates silver atoms when irradiated; (2) a non-photosensitive, reducible silver source; (3) a reducing agent (i.e., a developer) for silver ion; and (4) a binder.
The photosensitive material is generally photographic silver halide which must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver atoms (also known as silver specks, clusters, or nuclei) are generated by irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. It has long been understood that silver atoms (Ag.degree.) are a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions. For example, catalytic proximity can be accomplished by partial metathesis of the reducible silver source with a halogen-containing source (see, for example, U.S. Pat. No. 3,457,075); by coprecipitation of silver halide and the reducible silver source material (see, for example, U.S. Pat. No. 3,839,049); and other methods that intimately associate the photosensitive, photographic silver halide and the non-photosensitive, reducible silver source.
The non-photosensitive, reducible silver source is a material that contains silver ions. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates, have been proposed. U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image is generally not visible by ordinary means. Thus, the photosensitive emulsion must be further processed to produce a visible image. This is accomplished by the reduction of silver ions which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms, i.e., the latent image.
In a photothermographic element, the reducing agent for the organic silver salt, often referred to as a "developer," may be any material, preferably any organic material, that can reduce silver ion to metallic silver. At elevated temperatures, in the presence of the latent image, the non-photosensitive reducible silver source (e.g., silver behenate) is reduced by the reducing agent for silver ion. This produces a negative black-and-white image of elemental silver.
While conventional photographic developers such as methyl gallate, hydroquinone, substituted hydroquinones, hindered phenols, catechol, pyrogallol, ascorbic acid, and ascorbic acid derivatives are useful, they tend to result in very reactive photothermographic formulations and fog during preparation and coating of the photothermographic element. As a result, hindered bisphenol reducing agents have traditionally been preferred.
As the visible image in black-and-whim photothermographic elements is produced entirely by elemental silver (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable to reduce the cost of raw materials used in the emulsion and/or to enhance performance. For example, toning agents may be incorporated to improve the color of the silver image of the photothermographic element.
Another method of increasing the maximum image density in photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming or dye-releasing materials in the emulsion. Upon imaging, the dye-forming or dye-releasing material is oxidized, and a dye and a reduced silver image are simultaneously formed in the exposed region. In this way, a dye-enhanced silver image can be produced.
The imaging arts have long recognized the fields of photothermography and thermography as being clearly distinct from that of photography. Photothermographic and thermographic elements significantly differ from conventional silver halide photographic elements which require wet-processing.
In photothermographic and thermographic imaging elements, a visible image is created by heat as a result of the reaction of a developer incorporated within the element. Heat is essential for development and temperatures of over 100.degree. C. are routinely required. In contrast, conventional wet-processed photographic imaging elements require processing in aqueous processing baths to provide a visible image (e.g., developing and fixing baths) and development is usually performed at a more moderate temperature (e.g., 30.degree.-50.degree. C.).
In photothermographic elements only a small amount of silver halide is used to capture light and a different form of silver (e.g., silver behenate) is used to generate the image with heat. Thus, the silver halide serves as a catalyst for the development of the non-photosensitive, reducible silver source. In contrast, conventional wet-processed photographic elements use only one form of silver (e.g., silver halide) which, upon development, is converted to silver. Additionally, photothermographic elements require an amount of silver halide per unit area that is as little as one-hundredth of that used in a conventional wet-processed silver halide.
Photothermographic systems employ a light-insensitive silver salt, such as silver behenate, which participates with the developer in developing the latent image. In contrast, photographic systems do not employ a light-insensitive silver salt directly in the image-forming process. As a result, the image in photothermographic elements is produced primarily by reduction of the light-insensitive silver source (silver behenate) while the image in photographic black-and-white elements is produced primarily by the silver halide.
In photothermographic and thermographic elements, all of the "chemistry" of the system is incorporated within the element itself. For example, photothermographic and thermographic elements incorporate a developer (i.e., a reducing agent for the non-photosensitive reducible source of silver) within the element while conventional photographic elements do not. The incorporation of the developer into photothermographic elements can lead to increased formation of "fog" upon coating of photothermographic emulsions as compared to photographic emulsions. Even in so-called instant photography, developer chemistry is physically separated from the silver halide until development is desired. Much effort has gone into the preparation and manufacture of photothermographic and thermographic elements to minimize formation of fog upon coating, storage, and post-processing aging.
Similarly, in photothermographic elements, the unexposed silver halide inherently remains after development and the element must be stabilized against further development. In contrast, the silver halide is removed from photographic elements after development to prevent further imaging (i.e., the fixing step).
In photothermographic and thermographic elements the binder is capable of wide variation and a number of binders are useful in preparing these elements. In contrast, photographic elements are limited almost exclusively to hydrophilic colloidal binders such as gelatin.
Because photothermographic elements require thermal processing, they pose different considerations and present distinctly different problems in manufacture and use. In addition, the effects of additives (e.g., stabilizers, antifoggants, speed enhancers, sensitizers, supersensitizers, etc.) which are intended to have a direct effect upon the imaging process can vary depending upon whether they have been incorporated in a photothermographic or thermographic element or incorporated in a photographic element.
Distinctions between photothermographic and photographic elements are described in Imaging Processes and Materials (Neblette's Eighth Edition), J. Sturge et al. Ed., Van Nostrand Reinhold, New York, 1989, Chapter 9 and in Unconventional Imaging Processes, E. Brinckman et at, Ed., The Focal Press, London and New York, 1978, pp. 74-75.
Thermographic imaging constructions (i.e., heat-developable materials) processed with heat, and without liquid development, are widely known in the imaging arts and rely on the use of heat to help produce an image. Upon heating, typically in the range of about 60.degree.-225.degree. C., a reaction occurs only in the heated areas resulting in the formation of an image.
Thermographic elements whose image-forming layers are based on silver salts of long chain fatty acids, such as silver behenate, are also known. These elements generally comprise a support or substrate (such as paper, plastics, metals, glass, and the like) having coated thereon: (1) a thermally-sensitive reducible silver source; (2) a reducing agent for the thermally-sensitive reducible silver source (i.e., a developer); and (3) a binder. Upon heating, silver behenate is reduced by a reducing agent for silver ion such as methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechol, pyrogallol, ascorbic acid, ascorbic acid derivatives, leuco dyes, and the like, whereby an image comprised of elemental silver is formed.
Photothermographic and thermographic constructions are usually prepared by coating from solution and removing most of the coating solvent by drying. One common problem that exists with coating photothermographic systems is the formation of coating defects. Many of the defects and problems that occur in the final product can be attributed to structural changes within the coatings during the coating and drying processes. Among the problems that are known to occur during drying of polymeric film layers after coating is unevenness in the distribution of solid materials within the layer. Examples of specific types of coating defects encountered are "orange peel", "mottling", and "fisheyes". "Orange peel" is a fairly regular grainy surface that occurs on a dried, coated film usually because of the action of the solvent on the materials in the coating composition. "Fisheyes" are another type of coating problem, usually resulting from a separation of components during drying. There are pockets of different ingredients within the drying solution, and these pockets dry out into uneven coating anomalies. "Mottling" often occurs because of an unevenness in the removal of the solvent from the coating composition.
When a coating solution is dried at high speeds in an industrial oven, the resulting film often contains a motile pattern. This motile pattern is typically the result of surface tension gradients created by non-uniform drying conditions. Fluorochemical surfactants have been found to be particularly useful in coating applications to reduce mottle. When an appropriate fluorochemical surfactant is added to the coating solution, the surfactant holds the surface tension at a lower, but constant value. This results in a uniform film, free from mottle. Fluorochemical surfactants are used because organic solvents, such as 2-butanone (also known as methyl ethyl ketone or MEK), already have such low surface energies (24.9 dyne/cm) that hydrocarbon surfactants are ineffective. Copending U.S. patent application Ser. No. 08/104,888 (filed Aug. 10, 1993) describes the use of fluorochemical surfactants to reduce coating disuniformities such as motile, fisheyes and orange peel in photothermographic and thermographic elements. These fluorochemical surfactants are comprised of fluorinated terpolymers which are polymerization products of: (1) a fluorinated, ethylenically unsaturated monomer; (2) a hydroxyl-containing, ethylenically unsaturated monomer; and (3) a polar, ethylenically unsaturated monomer. The addition of these fluorochemical surfactants into the emulsion coatings gives rise to uniform optical densities which is highly desirable in medical diagnostic applications.
Since these fluorochemical surfactants act as surface active modifiers, the surface of the dried element has a slight tack due to the concentration of low molecular weight material at the surface. This tack may not present a problem when elements are manually removed from a container or cartridge; however, in an automated film-feeding apparatus the tack of the surface can Cause multi-films to be transported in the apparatus. The transportation of multiple films or elements can cause operational failure of the apparatus and can potentially damage internal mechanisms within the apparatus. At best, an operator has to open the apparatus to clear the jam, thereby resulting in loss of productivity which defeats the purpose of an automated system.
The addition of particulates, such as starch, titanium dioxide, zinc oxide, silica, and polyfluoroethylene polymeric beads are well known in the art as anti-blocking or slip agents. These types of particulates are translucent or opaque, thereby causing deteriorative effects on the image contrast.
The use of particulate matter in adhesive layers for anti-blocking characteristics is well known. A specific example of using organic polymeric beads with a narrow molecular weight distribution in an adhesive layer of a surprint color proof is described in U.S. Pat. No. 4,885,225. In this particular application, the size of the polymeric beads is kept small enough to become encapsulated into the adhesive when the proofing film is laminated to an opaque support; and thus, the beads have little or no effect on the visual properties of the final imaged proof.
The use of organic polymeric beads with a narrow molecular weight distribution in a protective layer of an overlap color proof is described in U.S. Pat. No. 5,258,261. The protective layer in this application is removed during the imaging process; and therefore, the beads would have no visual effect on the final image of the proof. Unlike liquid processed media that use polymeric beads in the topmost layer, photothermographic and thermographic elements typically do not remove the outermost layer in the imaging process.
The use of organic polymeric beads has also been shown to reduce the effects of Newton's rings when a film is contacted with reproduction media during the exposure process. A specific example of this application is described in U.S. Pat. No. 2,992,101.
Organic polymeric beads dispersed in a water-based receptive coating have also been shown to be useful in electrostatic transparencies imaged in plain paper copiers. Specific examples of this application is described in U.S. Pat. Nos. 5,310,595 and 4,869,955. In these applications the image is transferred onto the receptive layer containing the polymeric beads.